When a post office handles Alarge quantities of mail daily, it is essential that mechanical methods Bare used Cto maintain produ

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问题 When a post office handles Alarge quantities of mail daily, it is essential that mechanical methods Bare used Cto maintain production and insure Dprompt delivery of the mails.

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答案B错,应该改为“be used”。

解析 “essential”引导的从句应该用虚拟语气。从句的谓语动词应该是“(should)+动词原形”,所以应该把“are used”改为“be used”。
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